Lithium-ion secondary battery

ABSTRACT

A lithium ion secondary battery provided with both high weight energy density and good cycle characteristics (capacity retaining ratio during an extended use). A secondary battery comprising a negative electrode having, as a negative electrode active material, carbon and a lithium absorbing material that forms an alloy with lithium, the above active material having a layer structure, a positive electrode capable of absorbing and desorbing lithium ions, and an electrolyte disposed between the positive and negative electrodes, wherein the Li content in the lithium absorbing material layer in the negative electrode is between 31 and 67% at a discharge depth of 100%.

TECHNICAL FIELD

The present invention relates to a lithium ion secondary battery.

BACKGROUND ART

Lithium ion secondary batteries have been under development in recentyears, and, since their battery characteristics such as charge/dischargevoltage, charge/discharge cycle life characteristic and shelf lifecharacteristic are highly dependent on utilized electrode activematerials, the electrode active materials have been improved to enhancethe battery characteristics.

In the case where metal lithium is used as a negative electrode activematerial, it is possible to form a lightweight battery with high-energydensity. In this case, however, dendrites are formed on the surface ofthe lithium metal at the time of charging as the charge/discharge cycleis repeated. The dendrites may penetrate a separator and cause a shortcircuit, thus shortening the life of the battery.

In order to solve the problem, there has been proposed a lithiumsecondary battery utilizing as a negative electrode lithium absorbingmaterials that are electrochemically alloyed with lithium at the time ofcharging, such as aluminum, silicon and tin (Solid State Ionics,113-115, p57 (1998)).

The negative electrode using such lithium absorbing material has a highcapacity, and absorbed/desorbed lithium ions per unit volume are largein quantity. However, the negative electrode is progressively pulverizedor flakes off as the charge/discharge cycle is repeated due to theexpansion and contraction of the lithium absorbing material being anelectrode active material when absorbing and desorbing lithium ions,which results in the short charge/discharge cycle life.

Consequently, there has been proposed to use a graphite material thathas a reversible ability to absorb and desorb lithium ions as a negativeelectrode. The graphite material does not have the aforementionedpulverization problem, and combines relatively excellent cycleperformance and safety. However, since graphite absorbs Li in the formof LiC₆, it has a capacity of up to 372 mAh/g per unit of weight onlyand has a low weight energy density.

As a means of realizing high energy density by a combination of thelithium absorbing material and graphite material, it has been suggestedthat a carbon negative electrode could be coated with the lithiumabsorbing material. Since the capacity of the base graphite materialdoes not much decrease, a certain amount of capacity can be maintainedfor the whole of the negative electrode even when there is a decrease inthe capacity of a silicon layer. Consequently, cycle characteristics aremore improved than would be the case with the lithium absorbing materialalone. Nevertheless, there is still a decrease in the capacity of thesilicon layer because of the expansion and contraction of silicon, andtherefore, good cycle characteristics as when using a carbon materialonly cannot be obtained.

PROBLEMS THAT THE INVENTION IS TO SOLVE

It is therefore an object of the present invention to provide a lithiumion secondary battery provided with both high weight energy density andgood cycle characteristics

DISCLOSURE OF THE INVENTION

In accordance with the present invention, there is provided a lithiumion secondary battery comprising a positive electrode capable ofabsorbing and desorbing lithium ions and a negative electrode includinga first layer that is largely composed of carbon and a second layer thatcontains an element to be alloyed with lithium, wherein the lithiumcontent of the second layer is between 31 and 67 atomic-% at a dischargedepth of 100%. Incidentally, the discharge depth means the ratio ofdischarged capacity to dischargeable capacity. A discharge depth of 100%indicates a condition where a battery has been discharged completelywithout being stopped in the middle.

According to the present invention, the lithium content of the secondlayer is between 31 and 67 atomic-% at a discharge depth of 100%.Consequently, it is possible to effectively prevent the progress ofpulverization of the second layer due to expansion and contractioncaused by the absorption and desorption of lithium ions. Thus, cyclecharacteristics (charge/discharge cycle) can be remarkably improved.

According to another embodiment of the present invention, in theabove-described lithium ion secondary battery, the capacity of thenegative electrode is higher than that of the positive electrode.

Generally, a positive electrode active material is heavier than anegative electrode active material in weight. Therefore, in order toincrease energy density per unit of weight, it is desirable to raise therate of utilization of the positive electrode active material. With theaforementioned construction, the capacity of the negative electrode ishigher than that of the positive electrode. Thus, it is possible toincrease energy density per unit of weight.

Additionally, by setting the capacity of the negative electrode higherthan that of the positive electrode, it becomes possible to sufficientlysuppress the rise of anode or negative electrode potential caused byover discharge. As a result, over discharge characteristics can beimproved.

According to another embodiment of the present invention, in theabove-described lithium ion secondary battery, lithium in a quantitythat satisfies the following formulas (1) and (2) is electricallyconnected to the positive electrode or the negative electrode:Li=Cb(1=L _(c))+(M _(atom) ×L _(s)/(1−L _(s)))×Li _(capa)  (1)Li+Cat≦Cb+M _(atom) ×M _(capa)  (2)(wherein Li is the capacity or quantity of Li electrically connected tothe positive electrode or the negative electrode, Cb is the capacity ofactive material contained in the first layer of the negative electrode,Lc is the initial charging/discharging efficiency of the first layer ofthe negative electrode, M_(atom) is the number of atoms of activematerial contained in the second layer of the negative electrode, L_(s)is the Li content in the second layer of the negative electrode at adischarge depth of 100%, Li_(capa) is the capacity of a lithium atom,Cat is the capacity of the positive electrode, and M_(capa) is thecapacity of an atom of lithium absorbing material contained in thesecond layer of the negative electrode).

In the above-described lithium ion secondary battery, the lithiumabsorbing material contained in the second layer of the negativeelectrode may include at least one element selected from Si, Ge, In, Sn,Ag, Al and Pb.

Particularly, in the above-described lithium ion secondary battery, thelithium absorbing material contained in the second layer of the negativeelectrode may include Si and/or Sn.

According to yet another embodiment of the present invention, in theabove-described lithium ion secondary battery, the first layer of thenegative electrode includes at least one selected from graphite,fullerene, carbon nanotube, DLC (Diamond Like Carbon), amorphous carbon,and hard carbon.

According to yet another embodiment of the present invention, in theabove-described lithium ion secondary battery, the active material ofthe positive electrode includes at least one compound selected fromlithium cobalt oxide, lithium manganese oxide, and lithium nickel oxide.Examples of the compound are not limited to lithium cobaltate, lithiummanganate, and the like. In these compounds, elements, for example,titanium, silicon or the like may be substituted for part of elements,cobalt, manganese and nickel.

According to yet another embodiment of the present invention, in theabove-described lithium ion secondary battery, the active material ofthe positive electrode includes lithium manganate. It is known thatlithium manganate has excellent over discharge characteristics. In thecase where the above-described negative electrode is combined with thepositive electrode that contains lithium manganate, it is possible toimprove over discharge characteristics as well as over chargecharacteristics. Thus, the reliability of the battery is vastlyincreased.

Further, in accordance with the present invention, there is provided amethod for using a lithium ion secondary battery comprising a positiveelectrode capable of absorbing and desorbing lithium ions and a negativeelectrode including a first layer that is largely composed of carbon anda second layer that contains an element to be alloyed with lithium,wherein the lithium content in the second layer of the negativeelectrode is made between 31 and 67 atomic-% on completion of discharge.

According to the present invention, the lithium content in the secondlayer of the negative electrode is between 31 and 67 atomic-% at adischarge depth of 100%. Consequently, it is possible to effectivelyprevent the progress of pulverization of the second layer due toexpansion and contraction caused by the absorption and desorption oflithium ions. Thus, cycle characteristics can be remarkably improved.

According to another embodiment of the present invention, in theabove-described method for using the lithium ion secondary battery, thecapacity of the negative electrode is higher than that of the positiveelectrode.

Generally, a positive electrode active material is heavier than anegative electrode active material in weight. Therefore, in order toincrease energy density per unit of weight, it is desirable to raise therate of utilization of the positive electrode active material. With theaforementioned construction, the capacity of the negative electrode ishigher than that of the positive electrode. Thus, it is possible toincrease energy density per unit of weight.

Still further, in accordance with the present invention, there isprovided a method for manufacturing a lithium ion secondary batterycomprising a positive electrode capable of absorbing and desorbinglithium ions and a negative electrode, involving the step of, afterforming the negative electrode layers including a first layer that islargely composed of carbon and a second layer that contains an elementto be alloyed with lithium, adding lithium in a quantity that satisfiesthe following formulas (A) to (D) to the second layer of the negativeelectrode:Cb+M _(atom) ×M _(capa)>Cat  (A)0.31≦L _(s)≦0.67  (B)Li=Cb(1−L _(c))+(M _(atom) ×L _(s)/(1−L _(s)))×Li _(capa)  (C)Li+Cat≦Cb+M _(atom) ×M _(capa)  (D)(wherein Li is the capacity of Li electrically connected to the positiveelectrode or the negative electrode, Cb is the capacity of activematerial contained in the first layer of the negative electrode, L_(c)is the initial charge/discharge efficiency of the first layer of thenegative electrode, M_(atom) is the number of atoms of lithium absorbingmaterial being active material contained in the second layer of thenegative electrode, L_(s) is the Li content in the second layer of thenegative electrode at a discharge depth of 100%, Li_(capa) is thecapacity of a lithium atom, Cat is the capacity of the positiveelectrode, and M_(capa) is the capacity of an atom of lithium absorbingmaterial being active material contained in the second layer of thenegative electrode).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an example of a negativeelectrode of a nonaqueous electrolyte secondary battery according to thefirst and second embodiments of the present invention.

FIG. 2 is a graph showing the relation between the Li content in alithium absorbing material layer of the negative electrode at adischarge depth of 100% and the discharge capacity retention ratio afterthe 100th cycle.

FIG. 3 is a diagram for explaining the effect of an IR drop or droppingon the discharge capacity.

FIG. 4 is a cross-sectional view showing another example of a negativeelectrode of a nonaqueous electrolyte secondary battery according to thefirst and second embodiments of the present invention.

FIGS. 5(a) and 5(b) are diagrams for explaining the relation between thecapacities of positive and negative electrodes and the capacity ofadditional Li.

FIG. 6 is a graph showing the charge/discharge curve of a nonaqueouselectrolyte secondary battery according to the second embodiment of thepresent invention.

Incidentally, the reference numeral 1 a represents a collector. Thereference numeral 2 a represents a carbon layer. The reference numeral 3a represents a lithium absorbing material layer.

BEST MODE FOR CARRYING OUT THE INVENTION

A lithium ion secondary battery according to the present inventioncomprises a negative electrode having, for example, a lithium absorbingmaterial layer 3 a mainly composed of an element that can be alloyedwith Li on a carbon layer 2 a formed on the collector 1 a as shown inFIG. 1, and a positive electrode containing a lithium compound, Such asLiCoO₂, from which lithium ions can be electrochemically extracted.

In the case of the lithium secondary battery having the negativeelectrode according to the present invention, good cycle characteristicscannot be achieved if the battery is charged under the same condition aswith a battery having a carbonaceous or graphite negative electrode. Inother words, when setting a discharge voltage at 1 to 2.5 V as in thecase of a carbonaceous or graphite negative electrode (counterelectrode: metal lithium), most of lithium that can be extracted fromthe lithium absorbing material layer is drawn out. On this occasion, thevolume of the lithium absorbing material layer is reduced due to thedischarge of lithium absorbing material in the negative electrode.Consequently, the lithium absorbing material is pulverized. Besides, onthe occasion of absorption and desorption of Li in such negativeelectrode, the degrees of expansion and contraction of volume differbetween the carbon layer and the lithium absorbing material layer.Accordingly, stresses are produced, and the lithium absorbing materiallayer flakes off the carbon layer. The flaking and pulverization of thelithium absorbing material cause a serious deterioration in cyclecharacteristics.

Given this factor, in accordance with the present invention, the Licontent in the lithium absorbing material layer on completion ofdischarge is controlled so as to be within a range of 31 to 67 atomic-%.Thereby, Li remains in the lithium absorbing material layer even afterthe completion of discharging, which alleviates expansion andcontraction of the volume of the lithium absorbing material layer. As aresult, the stresses produced between the carbon layer and the lithiumabsorbing material layer are reduced. Thus, it is possible to overcomethe problem involved in the use of the aforementioned negativeelectrode. That is, the lithium absorbing material layer can beprevented from flaking off the carbon layer. For the above reason, goodcycle characteristics can be achieved.

In the following, a description will be given of the reason why the Licontent of the lithium absorbing material layer on completion ofdischarge is made between 31 and 67 atomic-%.

FIG. 2 is a graph showing the relation between the Li content in thelithium absorbing material layer of the negative electrode on completionof discharge (discharge depth: 100%) and the discharge capacityretention ratio after the 100th cycle. The examined lithium secondarybattery has a negative electrode that includes a carbon layer, a siliconlayer and lithium foil stacked on a collector in layers, and a positiveelectrode that contains lithium cobaltate as an active material. As canbe seen in FIG. 2, when the Li content is low (less than 30 atomic-%)and also high (more than or equal to 70 atomic-%), the dischargecapacity retention ratio is low. Looking at changes in the Li contentfrom a low value to a high value in FIG. 2, it will be noted that thedischarge capacity retention ratio is improved drastically after thepoint of 31 atomic-%. Meanwhile, looking at changes in the Li contentfrom a high value to a low value, it will be noted that the dischargecapacity retention ratio is improved drastically after the point of 67atomic-%.

This phenomenon can be explained as follows. That is, in the case wherethe Li content in the lithium absorbing material layer of the negativeelectrode is less than 30 atomic-% on completion of discharge, expansionand contraction associated with charge/discharge cycles are notsufficiently alleviated. Consequently, the pulverization and flaking ofthe lithium absorbing material layer are not sufficiently suppressed. Asa result, good cycle characteristics may not be achieved. On the otherhand, in the case where the Li content exceeds 67 atomic-%, presumably,the discharge capacity retention ratio falls by the effect of theso-called IR drop. In the following, a description will be made of adecline in the discharge capacity retention ratio due to the IR drop.

FIG. 3 shows an example of a discharge curve for a battery. Generally, abattery is designed so that its discharge stops at a prescribed voltage.Accordingly, the discharge capacity is determined. However, illpractice, the discharge sometimes stops at a voltage that has not beenup to a discharge end voltage for various reasons. This is called IRdrop. When the IR drop occurs, the actual discharge capacity of abattery, whose design discharge capacity is Kd, becomes Kc in FIG. 3,and the capacity corresponding to C1 is not to be discharged. Besides,the actual discharge capacity of a battery, whose design dischargecapacity is Kb, becomes Ka, and the capacity corresponding to C2 is notto be discharged. As can be seen in FIG. 3, C2 is large as compared toC1 because the behavior of the discharge curve differs widely betweenareas 1 and 2. In other words, the difference in the design dischargecapacity causes a large difference in the capacity not to be dischargeddue to the effect of the IR drop, namely, the difference between C1 andC2. Here, the discharge capacity is set low when the lithium absorbingmaterial layer of the negative electrode has a high Li content, namely,the design discharge capacity corresponds to Kb. On the other hand, thedischarge capacity is set high when the lithium absorbing material layerof the negative electrode has a low Li content, namely, the designdischarge capacity corresponds to Kd. As is described above, it wouldappear that in the case where the Li content in the lithium absorbingmaterial layer of the negative electrode exceeds 67 atomic-%, thedischarge capacity retention ratio falls considerably.

For the reason stated above, by setting the Li content in the lithiumabsorbing material layer of the negative electrode on completion ofdischarge to be within a range of 31 to 67 atomic-%, it is possible torealize a lithium ion secondary battery provided with both high weightenergy density and good cycle characteristics.

Incidentally, while silicon is used for the lithium absorbing materiallayer in the description of FIG. 2, silicon is given only as an example.The lithium absorbing material layer may be formed of active materialother than silicon, such as Ge, In, Sn, Ag, Al, and Pb, with the samerelation between the Li content in the lithium absorbing material layerof the negative electrode at a discharge depth of 100% and the dischargecapacity retention ratio after the 100th cycle. These lithium absorbingmaterials each have a different discharge potential than that of carbon,and show the discharge curve as shown in FIG. 3. Consequently, there ismade a difference in a capacity decrease due to the IR drop, and thedischarge capacity retention ratio falls considerably in the high Licontent value area in FIG. 3. Additionally, Si, Sn, Ge and Pb absorbabout 4.4 lithium atoms per atom, and are similar to one another inlithium absorbing/desorbing behavior. Therefore, they share a commonappropriate lithium content range in FIG. 2.

Besides, with respect to the relation between the Li content and thedischarge capacity retention ratio shown in FIG. 2, by setting the Licontent to be within a range of 31 to 67 atomic-% as in the case of thenegative electrode using silicon, it is possible to achieve a gooddischarge capacity retention ratio even if using other active materials.

There is no particular limitation upon the order of stacking the carbonlayer and the lithium absorbing material layer of the negative electrodeused in the present invention. The lithium absorbing material layer maybe formed first on the collector before forming the carbon layer. Inthis case, expansion and contraction of the volume of the lithiumabsorbing material layer are alleviated, and therefore, the stresses arereduced. Thus, the lithium absorbing material layer can be preventedfrom flaking off. Even when part of the lithium absorbing material layerflakes off the collector, unless the part also flakes off the carbonlayer, the capacity does not decrease since conductivity is securedthrough the carbon layer. Further, the carbon layers and the lithiumabsorbing material layers may be stacked alternately in layers to form amultitiered structure negative electrode. Incidentally, when the surfaceof the negative electrode is provided with lithium foil, it is desirablethat the layer directly underneath the foil should be the lithiumabsorbing material layer rather than the carbon layer.

[First Embodiment]

In the following, the first embodiment of the present invention will bedescribed in detail with reference to the drawings. FIG. 1 is across-sectional view showing a negative electrode of a nonaqueouselectrolyte secondary battery according to the first embodiment of thepresent invention.

The collector 1 a is an electrode for letting electric current out ofthe battery and drawing current into the battery from the outside on theoccasion of charging and discharging. The collector 1 a is only requiredto be (conductive) metal (foil), and metallic foil. made of, forexample, aluminum, copper, stainless steel, gold, tungsten, andmolybdenum may be employed. The collector 1 a is 5 to 25 μm inthickness.

The carbon layer 2 a is a negative electrode member that absorbs ordesorbs Li on the occasion of charging and discharging. The carbon layer2 a. is made of carbon capable of absorbing Li. For example, graphite,fullerene, carbon nanotube, DLC (Diamond Like Carbon), amorphous carbon,hard carbon may be used either individually or in mixtures with eachother. The carbon layer 2 a is 30 to 300 μm in thickness.

The lithium absorbing material layer 3 a is a negative electrode memberthat absorbs or desorbs Li on the occasion of charging and discharging.The lithium absorbing material layer 3 a is formed of metal, amorphousmetal, alloy or metal oxide, or two or more kinds of metal, amorphousmetal, alloy and/or metal oxide. The lithium absorbing material layer 3a may be a multitiered layer or a layer made of a mixture formed by, forexample, CVD, deposition or sputtering. Moreover, the lithium absorbingmaterial layer 3 a may be formed by applying metal particles, alloyparticles and metal oxide particles or a mixture of these with the useof a binder. Preferably, the lithium absorbing material layer 3 a ismade of metal, amorphous metal, or alloy, and includes at least one kindselected from the group of Si, Ge, In, Sn, Ag, Al and Pb. Although thereis no particular limitation on the thickness of the lithium absorbingmaterial layer 3 a, it may be, for example, 0.1 to 240 μm in thickness.With this film thickness, it is possible to achieve both high capacityand, good productivity for batteries. In addition, the lithium absorbingmaterial layer 3 a may be doped with boron, phosphorous, arsenic, andantimony to further lower resistivity.

As a construction similar to that of FIG. 1, the carbon layer 2 a andthe lithium absorbing material layer 3 a may be formed on both sides ofthe collector la as shown in FIG. 4.

A positive electrode available for the lithium secondary battery of thepresent invention may be formed in such a manner that composite oxidesLi_(X)MO₂ (M indicates at least one transition metal), for example,Li_(X)CoO₂, Li_(X)NiO₂, Li_(X)Mn₂O₄, Li_(X)MnO₃, Li_(X)Ni_(y)CO_(1-y)O₂or the like, a conductive substance such as carbon black and a bindersuch as polyvinylidene fluoride (PVDF) are dispersed and mixed in asolvent or a dispersion liquid such as N-methyl-2-pyrrolidone (NMP), andthe mixture is applied over a base substance such as aluminum foil.

Besides, 5 V-level active materials may be used as the positiveelectrode active material. In other words, active materials having aplateau region at a metallic lithium counter electrode voltage of notless than 4.5 V may be employed. For example, lithium-containingcombined oxides may be preferable. As examples of the availablelithium-containing combined oxides may be cited spinel structurelithium-manganese combined oxide, olivine structure lithium-containingcombined oxide, inverse spinel structure lithium-containing combinedoxide or the like. Lithium-containing combined oxide may be a compoundrepresented by the following general formula ( I ):Li_(a)(M_(x)Mn_(2-x-y)A_(y)) O₄  (I)(where 0<x, 0<y, x+y<2, and 0<a<1.2; M is at least one kind selectedfrom the group consisting of Ni, Co, Fe, Cr and Cu; A is at least onekind selected from Si and Ti).

With the use of such compound, it is possible to realize a superiorelectromotive force stably. Here, if M includes at least Ni, cyclecharacteristics and the like are further improved. It is desirable thatx should be set within a range so that the valence number of Mn becomes+3.9 or more. Further, when 0<y in the aforementioned compound, Mn issubstituted with a lighter element. Thereby, the amount of discharge perunit of weight increases, and high capacity can be achieved.

The lithium secondary battery of the present invention can bemanufactured in a manner as follows: after the negative electrode, whichhas a hydrophobic surface layer formed on the surface of lithium metalor a lithium alloy, and the above-mentioned positive electrode arestacked in layers via the separator, which is composed of a porous filmmade of fluorocarbon resin, polyolefin such as polypropylene andpolyethylene or the like, in dried air or an inert gas atmosphere, orthe stacked electrodes are wound, they are mounted in a battery can orsealed with a flexible film, which is composed of a layered product ofsynthetic resin and metallic foil, or the like.

An electrolytic solution is prepared by dissolving lithium salts in anaprotic organic solvent or the like. Examples of the organic solventinclude: ring carbonates such as propylenecarbonate (PC),ethylenecarbonate (EC), butylenecarbonate (BC), and vinylenecarbonate(VC); chain carbonates such as dimethylcarbonate (DMC), diethylcarbonate(DEC), ethylmethylcarbonate (EMC), and dipropylcarbonate (DPC);aliphatic carboxylic acid esters such as methyl formate, methyl acetate,ethyl propionate: γ-lactones such as γ-butyrolactone; chain ethers suchas 1,2-diethoxyethane (DEE) and ethoxymethoxyethane (EME); ring etherssuch as tetrahydrofuran or its derivatives and 2-methyltetrahydrofuran;dimethylsulfoxide; 1,3-dioxolane; formamide; acetamide;dimethylformamide; dioxolane; acetonitrile; propylnitrile; nitromethane;ethylmonoglyme; triester phosphate; trimethoxymetane; dioxolanederivatives; sulfolane; methylsulfolane; 1,3-dimethyl-2-imidazolidinone;3-methyl-2-oxazolidinone; propylenecarbonate derivatives; ethyl ether,1,3-propanesultone; anisole; and N-methylpyrrolidone, which may be usedeither individually or in mixtures of two or more kinds. As examples ofthe lithium salts dissolved in the organic solvents may be cited LiPF₆,LiAsF₆, LiAlCl₄, LiClO₄, LiBF₄, LiSbF₆, LiCF₃SO₃, LiCF₃CO₂, Li(CF₃SO₂)₂, LiN (CF₃SO₂)₂, LiB₁₀Cl₁₀, lower aliphatic lithiumcarboxylate, lithium chloroborane, 4-lithium phenyl borate, LiBr, LiI,LiSCN, LiCl and imides. In addition, a polymer electrolyte may be usedas a substitute for the electrolytic solution.

According to the first embodiment, in order to make the lithium contentin the lithium absorbing material layer 3 a between 31 and 67 atomic-%on completion of discharge, it may be proposed to set limits on the wayof charging/discharging. More specifically, the Li content in thelithium absorbing material layer 3 a is made between 31 and 67 atomic-%on completion of discharge by setting a limit on the discharge voltagebased on such battery voltage or negative electrode potential(reference: Li metal) that the Li content in the lithium absorbingmaterial layer 3 a becomes 31 to 67 atomic-% on completion of discharge,or by setting a limit on the discharge time based on such dischargecapacity that the Li content in the lithium absorbing material layer 3 abecomes 31 to 67 atomic-%.

EXAMPLE 1

In the following, the present invention will be more fully described byExample 1 according to the first embodiment of the present invention.

Copper foil, graphite having a thickness of 100 μm after compression andSi were used for the collector 1 a, carbon layer 2 a and lithiumabsorbing material layer 3 a of the negative electrode shown in FIG. 1.respectively. A lithium cobaltate mixture was employed for a positiveelectrode or cathode active material, and aluminum foil was employed fora cathode collector. A mixed solvent of ethylenecarbonate (EC) anddiethylcarbonate (DEC) (EC/DEC mixture ratio: 30 to 70 by volume), inwhich 1 mol/l (1M) of LiPF₆ was dissolved, was employed for anelectrolyte solution. With the aforementioned negative electrode,positive electrode, electrolyte solution and a separator, a cylindricalsecondary battery was formed. The electrodes were wound in a spiral.

The electric properties of the cylindrical secondary battery wereexamined by a charge/discharge tester. In order to provide Li content inthe lithium absorbing material layer 3 a, battery voltage limits shownin Table 1 were set in conducting tests.

As Comparative Example 1, tests were conducted on the same cylindricalsecondary battery as that prepared in Example 1 with the battery voltagelimits shown in Table 1.

As Comparative Example 2, a cylindrical secondary battery was preparedwith a negative electrode formed by applying the mixture of Siparticles, a binder and conductive adjuvant to a collector made of Cufoil, a separator, the aforementioned positive electrode and electrolytesolution, and tests were conducted on the battery with the batteryvoltage limits shown in Table 1.

In both Example 1 and Comparative Example 1, the batteries or cells weredisassembled after the initial discharge and part of the electrodes werecut out. Then, secondary ion mass spectrometry was carried out tomeasure the Li content in the lithium absorbing material layer 3 a.Results of the tests are shown in Table 1. The Li content of the lithiumabsorbing material layer 3 a was 53 atomic-% on completion of dischargein Example 1 and Comparative Example 2, while that in ComparativeExample 1 was 16 atomic-%.

Further, the batteries were continuously discharged and recharged untilthe 300th cycle in Example 1, and Comparative Examples 1 and 2 Thecapacity retention ratios after the 300th cycle are shown in Table 1.The capacity retention ratios were obtained using the following formula(II):(discharge capacity in each cycle)/(discharge capacity in the 10thcycle)   (II)

Compared with Comparative Example 1 in which the Li content of thelithium absorbing material layer 3 a was 16 atomic-%, in Example 1 wherethe Li content of the lithium absorbing material layer 3 a was 53atomic-% on completion of discharge, the capacity retention ratio afterthe 300th cycle rose by 65%. On the other hand, compared withComparative Example 2 in which Si particles were used for the negativeelectrode, in Example 1, the capacity retention ratio after the 300thcycle rose by 80%. As just described, Example 1 proved that cyclecharacteristics were enormously improved when the lithium absorbingmaterial layer 3 a contained Li on completion of discharge.

In addition, Table 1 shows the energy density (Wh/kg) per unit of weightafter the 300th cycle in respective Example 1, and Comparative Examples1 and 2. Referring to Table 1, the energy density per unit of weight was172 Wh/kg in Example l, and it was confirmed that a high energy densitywas achieved in Example 1. TABLE 1 Capacity Energy Li Content RetentionDensity Voltage Limit of Layer 3a Ratio After After After After After300th Charge Discharge Discharge 300th Cycle Voltage Voltage (atomic %)Cycle (Wh/kg) Example 1 4.2 V 3.6 V 53% 95.2% 172 Comparative 4.2 V 1.7V 16% 30.0% 63 Example 1 Comparative 4.2 V 3.6 V 53% 15.1% 27 Example 2

EXAMPLE 2

In the following, the present invention will be more fully described byExample 2 according to the first embodiment of the present invention.

Copper foil, graphite having a thickness of 100 μm after compression andSi were employed for the collector 1 a, carbon layer 2 a and lithiumabsorbing material layer 3 a of the negative electrode shown in FIG. 1,respectively. A lithium cobaltate mixture was employed for a positiveelectrode or cathode active material, and aluminum foil was employed fora cathode collector. A mixed solvent of ethylenecarbonate (EC) anddiethylcarbonate (DEC) (EC/DEC mixture ratio: 30 to 70 by volume), inwhich 1 mol/l (1M) of LiPF₆ was dissolved, was employed for anelectrolyte solution.

With the aforementioned negative electrode, positive electrode,electrolyte solution and a separator, a cylindrical secondary batterywas produced. The electrodes were wound in a spiral.

The electric properties of the cylindrical secondary battery wereexamined by a charge/discharge tester. In order to provide Li content inthe lithium absorbing material layer 3 a, discharge capacity limitsshown in Table 2 were set in conducting tests.

As Examples 3 and 4, tests were conducted on the same secondarybatteries as that prepared in Example 2 with the discharge capacitylimits shown in Table 2. In addition, as Comparative Example 3, testswere conducted on the same secondary battery as that prepared in Example2 with the discharge capacity limits shown in Table 2.

In all the Examples 2, 3 and 4 and Comparative Examples 3, 4 and 5, thebatteries or cells were disassembled after the initial discharge andpart of the electrodes were cut out. Then, secondary ion massspectrometry was carried out to measure the Li content in the lithiumabsorbing material layer 3 a. Results of the tests are shown in Table 2.The Li content of the lithium absorbing material layer 3 a was between49 to 63 atomic-% on completion of discharge in each of Examples 2, 3and 4, while those in Comparative Examples 3 and 4 were as low as 16atomic-% and 27 atomic-%, respectively. Besides, the Li content of thelithium absorbing material layer 3 a was as high as 73 atomic-% inComparative Example 5.

Further, the batteries were continuously discharged and recharged untilthe 300th cycle in Examples 2, 3 and 4, and Comparative Examples 3, 4and 5. The capacity retention ratios after the 300th cycle are shown inTable 2. The capacity retention ratios were obtained using the formula(II).

While the capacity retention ratio after the 300th cycle was 30% inComparative Example 3 where there was no discharge capacity limit, inExamples 2, 3 and 4 and Comparative Example 5 where the Li content ofthe lithium absorbing material layer 3 a was over 49 atomic-% oncompletion of discharge, the capacity retention ratio after the 300thcycle exceeded 94% and rose by more than 64%. On the other hand, inComparative Example 4 where the Li content of the lithium absorbingmaterial layer 3 a was 27 atomic-% on completion of discharge, thecapacity retention ratio was 45%, which was more than 49% below those ofExamples 2, 3 and 4. As just described, Examples 2, 3 and 4 proved thatcycle characteristics were enormously improved when the lithiumabsorbing material layer 3 a had a Li content according to the firstembodiment on completion of discharge.

In addition, Table 1 shows the energy density (Wh/kg) per unit of weightafter the 300th cycle in respective Examples 2, 3 and 4 and ComparativeExamples 3, 4 and 5. Referring to Table 2, the energy density per unitof weight was between 159 and 177 Wh/kg in Examples 2, 3 and 4, and was69 Wh/kg or more higher as compared to Comparative Examples 3 and 4. Onthe other hand, in Comparative Example 5 where the capacity retentionratio after the 300th cycle was 94%, the energy density was 130 Wh/kg,and sufficient energy density was not achieved. As Examples 2, 3 and 4show, it was confirmed that a battery provided with both high weightenergy density (Wh/kg) and good cycle characteristics could be obtainedwhen the lithium absorbing material layer 3 a had a Li content of 31 to67 atomic-% on completion of discharge. TABLE 2 Capacity Energy LiContent Retention Density Discharge Condition of Layer Ratio AfterDischarge 3a After After 300th Time Discharge Discharge 300th CycleLimit Capacity (atomic %) Cycle (Wh/kg) Example 2 3.1 h 370 mAh 49%95.3% 177 Example 3   3 h 360 mAh 53% 95.5% 172 Example 4 2.8 h 330 mAh63% 96.2% 159 Comparative No Limit 420 mAh 16% 30.1% 63 Example 3Comparative 3.3 h 400 mAh 27% 45.0% 90 Example 4 Comparative 2.3 h 280mAh 73% 94.1% 130 Example 5[Second Embodiment]

According to the first embodiment, the discharge has to be terminatedwhen the discharge voltage reaches a specific limit or when a specificperiod of discharge time has passed so that the Li content in thelithium absorbing material layer of the negative electrode is between 31 and 67 atomic-% on completion of discharge. In this view, the firstembodiment is impractical. Meanwhile, in accordance with the secondembodiment, it is realized that the Li content in the lithium absorbingmaterial layer of the negative electrode is between 31 and 67 atomic-%even when the discharge is carried on without being stopped in themiddle until it reaches a depth of 100%, that is, the battery iscompletely discharged. Herein, the discharge depth means the ratio ofdischarged capacity to dischargeable capacity.

According to the second embodiment, in order that the Li content in thelithium absorbing material layer 3 a is between 31 and 67 atomic-% at adischarge depth of 100%, the electrodes are designed so as to satisfythe conditions as follows:

-   Condition (1)—the capacity of the negative electrode is designed to    be higher than that of the positive electrode-   Condition (2)—Li is added to the positive or negative electrode so    that the Li content in the lithium absorbing material layer 3 a is    between 31 and 67 atomic-%-   Condition (3)—the capacities of the positive and negative electrodes    satisfy the following formula (III):    positive electrode capacity≦negative electrode capacity−additional    Li capacity  (III)

The above conditions can be represented using parameters as follows:Condition (1)−Cb+Matom×Mcapa>Cat  (IV)Condition (2)−Li=Cb(1−Lc)+Matom×Ls/(1−Ls)×Licapa  (V)Condition (3)−Li+Cat≦Cb+Matom×Mcapa  (VI)(in formulas (IV) to (VI), Cb is the capacity of active materialcontained in the carbon layer 2a, Matom is the number of atoms oflithium absorbing material M contained in the lithium absorbing materiallayer 3 a, Mcapa is the capacity of an atom of lithium absorbingmaterial M contained in the lithium absorbing material layer 3 a, Licapais the capacity of an atom of Li, Li is the capacity of additional Li,Cat is the capacity of the positive electrode, Lc is the initialcharge/discharge efficiency of the carbon layer 2 a, and Ls is Licontent in the lithium absorbing material layer 3 a at a discharge depthof 100% (0.31 atomic-%<Ls<0.67 atomic-%)).

In the following, a description will be made of the reason why the Licontent in the lithium absorbing material layer can be made between 31and 67 atomic-% at a discharge depth of 100% according to theabove-described battery design referring to FIG. 5. As an example offormula (VI), the case where Li+Cat=Cb+Matom×Mcapa will be explained.

FIG. 5(a) shows the initial state of a battery that satisfies the aboveconditions. First, the electrodes are designed so that the capacity ofthe negative electrode is higher than that of the positive electrode tomeet condition (1). Then, in order to meet condition (3), Li in aquantity corresponding to the difference between the capacity of thenegative electrode and that of the positive electrode, Cb(1−Lc)+Matom×Ls/(1−Ls)×Licapa, is added to the negative electrode. Onthis occasion, in order to meet condition (2), the capacity of added Liis made to coincide with the sum of the irreversible capacity of thecarbon layer (Cb (1−Lc)) and the capacity of Li to be left in thelithium absorbing material layer at a discharge depth of 100%((Matom×Ls/(1−Ls)×Licapa) corresponding to 31 to 67 atomic-% of thecapacity of the lithium absorbing material layer of the negativeelectrode).

FIG. 5(b) shows the charged state of the above battery. When the batteryis discharged to a discharge depth of 100%, Li in a quantitycorresponding to the quantity or capacity of Li which moved from thepositive electrode to the negative electrode at the time of chargingmoves from the negative electrode to the positive electrode. Thus, thebattery is restored to the initial state shown in FIG. 5(a).Consequently, according to the above-described battery design, it ispossible to leave a Li content of 31 to 67 atomic-% in the lithiumabsorbing material layer even if the battery is discharged to adischarge depth of 100% after the charged state shown in FIG. 5(b).Incidentally, Li of non-irreversible capacity that remains in thenegative electrode is present in the lithium absorbing material layer.This is because the discharge potential of the carbon layer is lowerthan that of the lithium absorbing material layer, and Li that hasabsorbed in the carbon layer moves to the positive electrode first.

There are some conventional techniques for adding Li to the negativeelectrode (e.g. Japanese Patent Application laid open No. HEI11-288705). However, the addition of Li in the conventional techniquesis solely aimed at compensation for Li corresponding to the irreversiblecapacity of the carbon layer of the negative electrode. Therefore, at adischarge depth of 100%, the Li content that remains in the negativeelectrode may be normally 10%, and less than or comparable to 20% sincethe Li content higher than that is unfavorable for achieving high weightenergy density, that is, the object of the present invention. On theother hand, according to the present invention, the addition of Li ismade in order to control the Li content of the lithium absorbingmaterial layer to be 31 to 67 atomic-% at a discharge depth of 100% aswell as compensating for Li corresponding to the irreversible capacitydifferently from the conventional techniques.

FIG. 6 is a graph showing an example of characteristics of the secondarybattery whose electrodes has been designed so as to satisfy the aboveformulas (IV) to (VI). As can be seen in FIG. 6, the negative electrodehas a Li content corresponding to the capacity of additional Li evenafter the discharge. Herewith, according to the battery design thatsatisfies the above conditions (1) to (3), it is possible to produce abattery in which the Li content in the lithium absorbing material layer3 a is between 31 and 67 atomic-% at a discharge depth of 100%.

Incidentally, in this embodiment, the same positive electrode, negativeelectrode, separator and electrolytic solution as previously describedfor the first embodiment may be utilized. Besides, in the case wherelithium manganate is employed as a positive electrode or cathode activematerial, it is possible to produce a battery provided with excellentover charge characteristics as well as over discharge characteristics bythe synergistic effect of the positive electrode and the negativeelectrode formed according to the battery design that satisfies theabove formulas (IV) to (VI).

EXAMPLE 5, 6 and 7

In the following, the present invention will be more fully described byExamples 5, 6 and 7 according to the second embodiment of the presentinvention.

In Example 5, a battery was formed based on the design of the electrodecapacities that satisfied the above formulas (IV) to (VI).

Copper foil, graphite having a thickness of 100 μm after compression andSi were used for the collector 1 a, carbon layer 2 a and lithiumabsorbing material layer 3 a of the negative electrode shown in FIG. 1,respectively. After forming the lithium absorbing material layer 3 a, Liin a quantity shown in Table 3 was deposited thereon to make theaddition of Li. A lithium cobaltate mixture was employed for a positiveelectrode or cathode active material, and aluminum foil was employed fora cathode collector. A mixed solvent of ethylenecarbonate (EC) anddiethylcarbonate (DEC) (EC/DEC mixture ratio: 30 to 70 by volume), inwhich 1 mol/l (1M) of LiPF₆ was dissolved, was employed for anelectrolyte solution.

In Example 6, after forming the positive electrode, it was plated withLi to make the addition of Li differently from Example 5. Otherwise, acylindrical secondary battery was formed based on the same electrodedesign as shown in Table 3 and in the same manner as in Example 5.

In Example 7, after forming the lithium absorbing material layer 3 a, Lifoil was attached thereon to make the addition of Li differently fromExample 5. Otherwise, a cylindrical secondary battery was formed basedon the same electrode design as shown in Table 3 and in the same manneras in Example 5.

In comparative Example 6, the electrodes were designed as shown in Table3, and a cylindrical secondary battery was formed with the samematerials and in the same manner as in Example 5 based on the electrodedesign shown in Table 3.

The electric properties of the above-described cylindrical secondarybatteries were examined by a charge/discharge tester. In all theExamples 5, 6 and 7 and Comparative Example 6, charging/discharging wascarried out from 2.5 to 4.2 V.

In Examples 5, 6 and 7 and Comparative Example 6, the batteries or cellswere disassembled after the initial discharge and part of the electrodeswere cut out. Then, secondary ion mass spectrometry was carried out tomeasure the Li content-in the lithium absorbing material layer 3 a.Results of the tests are shown in Table 4. The Li content of the lithiumabsorbing material layer 3 a was 60 atomic-% at a discharge depth of100% in Examples 5, 6 and 7, while that in Comparative Example 6 was 16atomic-%.

In addition, part of the same electrodes as used in Examples 5, 6 and 7and Comparative Example 6 were cut out into a circular form with adiameter of 1 cm. After that, coin- shaped batteries were produced withthe use of Li metal as counter electrodes. Then, the positive electrodeswere allowed to charge/discharge from 2.5 to 4.3 V, and the negativeelectrodes were allowed to charge/discharge from 2.5 to 0 V at 0.1 mA.In the initial charge/discharge, a capacity of 5 mAh was observed at 4.3V with respect to each positive electrode of Examples 5, 6 and 7 andComparative Example 6. On the other hand, with respect to the negativeelectrodes, a capacity of 6.25 mAh was observed at 0 V in Examples 5, 6and 7, while a capacity of 5 mAh was observed in Comparative Example 6.

Further, the batteries were continuously discharged and recharged untilthe 300th cycle in Examples 5, 6 and 7 and Comparative Example 6. Thecapacity retention ratios after the 300th cycle are shown in Table 4.The capacity retention ratios were obtained using formula ( H ).

Compared with Comparative Example 6 in which the Li content of thelithium absorbing material layer 3 a was 16 atomic-% at a dischargedepth of 100%, in Examples 5, 6 and 7 where the Li content of thelithium absorbing material layer 3 a was 60 atomic-% at a dischargedepth of 100%, the capacity retention ratio after the 300th cycle roseby more than 64%. As just described, Examples 5, 6 and 7 proved thatcycle characteristics were enormously improved by controlling the Licontent in the lithium absorbing material layer 3 a to be 31 to 67atomic-% at a discharge depth of 100%.

Besides, Table 4 shows the weight energy density (Wh/kg) after the 300thcycle in respective Examples 5, 6 and 7 and Comparative Example 6.Referring to Table 4, the weight energy density was 169 Wh/kg in Example5, 168 Wh/kg in Example 6, and 169 Wh/kg in Example 7. That is, theweight energy density was improved by 113 Wh/kg or more in Examples 5, 6and 7 as compared to Comparative Example 6. Thus, it was confirmed thata high energy density was achieved in Examples 5, 6 and 7. TABLE 3Positive Electrode Negative Electrode Additional Li Capacity CapacityCapacity Example 5 500 mAh 625 mAh 125 mAh Example 6 500 mAh 625 mAh 125mAh Example 7 500 mAh 625 mAh 125 mAh Comparative 500 mAh 500 mAh  0 mAhExample 6

TABLE 4 Li Content of Capacity Weight Energy Layer 3a After RetentionRatio Density After Discharge After 300th 300th Cycle (atomic %) Cycle(Wh/kg) Example 5 60% 95.0% 169 Example 6 60% 94.5% 168 Example 7 60%94.8% 169 Comparative 16% 30.1% 55 Example 6

EXAMPLES 8, 9 and 10

In the following, the present invention will be more fully described byExamples 8, 9 and 10 according to the second embodiment of the presentinvention.

In Example 8, a battery was formed based on the design of the electrodecapacities as show in FIG. 5, which satisfied the above formulas (IV) to(VI). TABLE 5 Positive Electrode Negative Electrode Additional LiCapacity Capacity Capacity Example 8 500 mAh 601 mAh 101 mAh Example 9500 mAh 601 mAh 101 mAh Example 10 500 mAh 601 mAh 101 mAh Comparative500 mAh 500 mAh  0 mAh Example 7

The electrodes were prepared based on the design of the electrodecapacities shown in Table 5, and a battery was formed. Copper foil,graphite having a thickness of 100 μm after compression and Si were usedfor the collector 1 a, carbon layer 2 a and lithium absorbing materiallayer 3 a of the negative electrode shown in FIG. 1, respectively. Afterforming the lithium absorbing material layer 3 a, Li in a quantity shownin Table 5 was deposited thereon to make the addition of Li. A lithiumcobaltate mixture was employed for a positive electrode or cathodeactive material, and aluminum foil was employed for a cathode collector.A mixed solvent of ethylenecarbonate (EC) and diethylcarbonate (DEC)(EC/DEC mixture ratio: 30 to 70 by volume), in which 1 mol/l (1M) ofLiPF₆ was dissolved, was employed for an electrolyte solution.

In Example 9, Sn was employed as a constituent element of the lithiumabsorbing material layer 3 a instead of Si used in Example 8. Otherwise,a cylindrical secondary battery was formed based on the same electrodedesign as shown in Table 5 and in the same manner as in Example 8.

In Example 10, Ge was employed as a constituent element of the lithiumabsorbing material layer 3 a instead of Si used in Example 8. Otherwise,a cylindrical secondary battery was formed based on the same electrodedesign as shown in Table 5 and in the same manner as in Example 8.

In comparative Example 7, the electrodes were designed as shown in Table5, and a cylindrical secondary battery was formed with the samematerials and in the same manner as in Example 8 based on the electrodedesign shown in Table 5.

The electric properties of the above-described cylindrical secondarybatteries were examined by a charge/discharge tester. In all theExamples 8, 9 and 10 and Comparative Example 7, charging/discharging wascarried out from 2.5 to 4.2 V.

In Examples 8, 9 and 10 and Comparative Example 7, the batteries orcells were disassembled after the initial discharge and part of theelectrodes were cut out. Then, secondary ion mass spectrometry wascarried out to measure the Li content in the lithium absorbing materiallayer 3 a. Results of the tests are shown in Table 6. The Li content oftile lithium absorbing material layer 3 a was 57 atomic-% at a dischargedepth of 100% in Examples 8, 9 and 10, while that in Comparative Example7 was 17 atomic-%.

In addition, part of the same electrodes as used in Examples 8, 9 and 10and Comparative Example 7 were cut out into a circular form with adiameter of 1 cm. After that, coin-shaped batteries were produced withthe use of Li metal as counter electrodes. Then, the positive electrodeswere allowed to charge/discharge from 2.5 to 4.3 V, and the negativeelectrodes were allowed to charge/discharge from 2.5 to 0 V at 0.1 mA.In Examples 8, 9 and 10 and Comparative Example 7, a capacity of 5 mAhwas observed with respect to the positive electrodes. On the other hand,with respect to the negative electrodes, a capacity of 6.01 mAh wasobserved in Examples 8, 9 and 10, while a capacity of 5 mAh was observedin Comparative Example 7.

Further, the batteries were continuously discharged and recharged untilthe 300th cycle in Examples 8, 9 and 10 and Comparative Example 7. Thecapacity retention ratios after the 300th cycle are shown in Table 6.The capacity retention ratios were obtained using formula (II).

Compared with Comparative Example 7 in which the Li content of thelithium absorbing material layer 3 a was 17 atomic-% at a dischargedepth of I 00%, in Examples 8, 9 and 10 where the Li content of thelithium absorbing material layer 3 a was 57 atomic-% at a dischargedepth of 100%, the capacity retention ratio after the 300th cycle roseby more than 64%. As just described, Examples 8, 9 and 10 proved thatcycle characteristics were enormously improved when the lithiumabsorbing material layer 3 a contained Li at a discharge depth of 100%.

Besides, Table 6 shows the weight energy density (Wh/kg) after the 300thcycle in respective Examples 8, 9 and 10 and Comparative Example 7.Referring to Table 6, the weight energy density was 168 Wh/kg in Example8, 169 Wh/kg in Example 9, and 170 Wh/kg in Example 10. That is, theweight energy density was improved by 1 13 Wh/kg or more in Examples 8,9 and 10 as compared to Comparative Example 7. Thus, it was confirmedthat a high energy density was achieved in Examples 8, 9 and 10. TABLE 6Li Content of Weight Energy Layer 3a After Capacity Density AfterDischarge Retention Ratio 300th Cycle (atomic %) After 300th Cycle(Wh/kg) Example 5 57% 94.3% 168 Example 6 57% 94.7% 169 Example 7 57%95.2% 170 Comparative 17% 30.2% 55 Example 6

EXAMPLE 11

In the following, the present invention will be more fully described byExample 11 according to the second embodiment of the present invention.In Example 11, the capacities of the electrodes were designed as shownin Table 7 so as to meet conditions (1), (2) and (3) of the secondembodiment. TABLE 7 Positive Electrode Negative Electrode Additional LiCapacity Capacity Capacity Example 11 500 mAh 625 mAh 125 mAh Example 12500 mAh 625 mAh 125 mAh Comparative 500 mAh 500 mAh  0 mAh Example 8

The electrodes were prepared based on the design of the electrodecapacities shown in Table 7, and a battery was formed. Copper foil,graphite having a thickness of 100 μin after compression and Si wereused for the collector 1 a, carbon layer 2 a and lithium absorbingmaterial layer 3 a of the negative electrode shown in FIG. 1,respectively. After forming the lithium absorbing material layer 3 a, Liin a quantity shown in Table 7 was deposited thereon to make theaddition of Li. A lithium cobaltate mixture was employed for a positiveelectrode or cathode active material, and aluminum foil was employed fora cathode collector. A mixed solvent of ethylenecarbonate (EC) anddiethylcarbonate (DEC) (EC/DEC mixture ratio: 30 to 70 by volume), inwhich 1 mol/l (1M) of LiPF₆ was dissolved, was employed for anelectrolyte solution.

EXAMPLE 12

In Example 12, a lithium manganate mixture was employed for a positiveelectrode active material instead of what was used in Example 11.Otherwise, a cylindrical secondary battery was formed based on the sameelectrode design as shown in Table 7 and in the same manner as inExample 11.

COMPARATIVE EXAMPLE 8

In comparative Example 8, the electrodes were designed as shown in Table7, and a cylindrical secondary battery was formed with the samematerials and in the same manner as in Example 11 based on the electrodedesign shown in Table 8.

In all the Examples 11 and 12 and Comparative Example 8, the batterieswere discharged and recharged repeatedly through several numbers ofcycles under a constant current of 0.6 A on the condition that thecharge final voltage be 4.2 V and the discharge final voltage be 2.5 V.After the discharge of the 10th cycle (the discharge capacity in the10th cycle will be denoted by {circle over (1)}), the batteries weretaken out, and allowed to discharge to 0 V with a resistance load of1KΩ. Then, the batteries were left as they were for two weeks. Afterthat, the batteries were charged up to the charge final voltage 4.2 Vunder a constant current of 0.6 A, and subsequently, discharged to thedischarge final voltage 2.5 V under a constant current of 0.6 A. Thedischarge capacity in this case will be denoted by {circle over (2)}.Further, in Example 12 and Comparative Example 8, the batteries weredischarged and recharged repeatedly under a constant current of 0.6 A onthe condition that the charge final voltage be 4.2 V and the dischargefinal voltage be 2.5 V. On the occasion of the charge of the 11th cycle,the charge final voltage was set to 5.0 V. Then, the batteries were leftas they were for two weeks. After that, the batteries were discharged to2.5 V, and subsequently, charged up to the charge final voltage 4.2 Vunder a constant current of 0.6 A. Thereafter, the battery wasdischarged to the discharge final voltage 2.5 V under a constant currentof 0.6 A. The discharge capacity in this case will be denoted by {circleover (3)}.

In Examples 11 and 12 and Comparative Example 8, the batteries or cellswere disassembled after the initial discharge and part of the electrodeswere cut out. Then, secondary ion mass spectrometry was carried out tomeasure the Li content in the lithium absorbing material layer 3 a.Results of the tests are shown in Table 8. The Li content of the lithiumabsorbing material layer 3 a was 60 atomic-% in Examples 11 and 12,while that in Comparative Example 8 was 16 atomic-%.

Table 8 shows the capacity retention ratio (%) to the discharge capacityin the 10th cycle after a discharge of 0 V in respective Examples 11 and12, and Comparative Example 8. Compared with Comparative Example 8 inwhich the Li content of the lithium absorbing material layer 3 a was 16atomic-% on completion of discharge, in Examples 11 and 12 where the Licontent of the lithium absorbing material layer 3 a was 60 atomic-% oncompletion of discharge, the capacity retention ratio after a dischargeof 0 V rose by more than 26%. Incidentally, the above capacity retentionratios (%) were calculated using the following formula (*VI):{circle over (2)}/{circle over (1)}×100=capacity retention ratio(%)  (*VII)

In Examples 11 and 12, improvement in over discharge characteristics wasachieved because the negative electrode capacity>the positive electrodecapacity, and therefore a rise in the potential of the negativeelectrode caused by the over discharge could be sufficiently suppressed.As just described, Examples 11 and 12 proved that over dischargecharacteristics were enormously improved.

Besides, Table 8 shows the capacity retention ratio (%) to the dischargecapacity in the 10th cycle after a charge of 5 V in Example 12 andComparative Example 8. Compared with Comparative Example 8 in which theLi content of the lithium absorbing material layer 3 a was 16 atomic-%on completion of discharge, in Example 12, the capacity retention ratioafter a charge of 5 V was 90.9%, and rose by more than 15%. Thus, it wasconfirmed that excellent over discharge characteristics were achieved inExample 12 where lithium manganate was employed for the positiveelectrode. Incidentally, the above capacity retention ratios (%) werefound out using the following formula (*VIII):{circle over (3)}/{circle over (1)}×100=capacity retentionratio(%)  (*VIII) TABLE 8 Capacity Capacity Retention Ratio to RetentionRatio to Discharge Discharge Li Content of Capacity in 10th Capacity in10th Layer 3a After Cycle After 0 V Cycle After 5 V Discharge Discharge(%) Charge (%) (atomic %) (*VII) (*VIII) Example 11 60% 94.8% — Example12 60% 94.2% 90.7% Comparative 16% 62.0% 60.1% Example 8

EXAMPLE 13

In the following, the present invention will be more fully described byExample 13 according to the second embodiment of the present invention.In Example 13, the capacities of the electrodes were designed as shownin Table 9 so as to satisfy the above formulas (IV) to (VI) forproducing a battery. TABLE 9 Positive Electrode Negative ElectrodeAdditional Li Capacity Capacity Capacity Example 13 500 mAh 580 mAh 80mAh Comparative 500 mAh 500 mAh  0 mAh Example 9

The electrodes were prepared based on the design of the electrodecapacities shown in Table 9, and a battery was formed. Copper foil, hardcarbon having a thickness of 100 μm after compression and Si were usedfor the collector 1 a, carbon layer 2 a and lithium absorbing materiallayer 3 a of the negative electrode shown in FIG. 1, respectively. Afterforming the lithium absorbing material layer 3 a, Li in a quantity shownin Table 9 was deposited thereon to make the addition of Li. A spinalstructure lithium-manganese combined oxide (LiNi_(0.5)Mn_(1.5)O₄)mixture having a plateau region at a metallic lithium counter electrodevoltage of not less than 4.5 V was employed for a positive electrode orcathode active material, and aluminum foil was employed for a cathodecollector. A mixed solvent of ethylenecarbonate (EC) anddiethylcarbonate (DEC) (EC/DEC mixture ratio: 30 to 70 by volume), inwhich 1 mol/l (1M) of LiPF₆ was dissolved, was employed for anelectrolyte solution.

In comparative Example 9, the electrodes were designed as shown in Table9, and a cylindrical secondary battery was formed based oil theelectrode design shown in Table 9.

The electric properties of the above-described cylindrical secondarybatteries were examined by a charge/discharge tester. In Example 13 andComparative Example 9, the batteries were allowed to charge/dischargefrom 2.5 to 4.75 V.

In both Example 13 and Comparative Example 9, the batteries or cellswere disassembled after the initial discharge and part of the electrodeswere cut out. Then, secondary ion mass spectrometry was carried out tomeasure the Li content in the lithium absorbing material layer 3 a.Results of the tests are shown in Table 10. The Li content of thelithium absorbing material layer 3 a was 53 atomic-% at a dischargedepth of 100% in Example 13, while that in Comparative Example 9 was 16atomic-%.

Besides, part of the same electrodes as used in Example 13 andComparative Example 9 were cut out into a circular form with a diameterof 1 cm. After that, coin-shaped batteries were produced with the use ofLi metal as counter electrodes. Then, the positive electrodes wereallowed to charge/discharge from 2.5 to 4.85 V, and the negativeelectrodes were allowed to charge/discharge from 2.5 to 0 V at 0.1 mA.In Example 13 and Comparative Example 9, a capacity of 5 mAh wasobserved at 4.85 V with respect to the positive electrodes. On the otherhand, with respect to the negative electrodes, a capacity of 5.8 mAh wasobserved at 0 V in Example 13, while a capacity of 5 mAh was observed inComparative Example 9.

Further, the batteries were continuously discharged and recharged untilthe 300th cycle in Example 13 and Comparative Example 9. The capacityretention ratios after the 300th cycle are shown in Table 10. Thecapacity retention ratios were obtained using formula ( II).

Compared with Comparative Example 9 in which the Li content of thelithium absorbing material layer 3 a was 16 atomic-% at a dischargedepth of 100%, in Example 13 where the Li content of the lithiumabsorbing material layer 3 a was 53 atomic-% at a discharge depth of100%, the capacity retention ratio after the 300th cycle rose by morethan 60%. As just described, Example 13 proved that cyclecharacteristics were enormously improved when the lithium absorbingmaterial layer 3 a contained Li at a discharge depth of 100%.

Besides, Table 10 shows the weight energy density (Wh/kg) after the300th cycle in Example 13 and Comparative Example 9. Referring to Table10, the weight energy density was 182 Wh/kg in Example 13, and it wasimproved by 121 Wh/kg or more as compared to Comparative Example 9.Thus, it was confirmed that a high energy density was achieved inExample 13. TABLE 10 Li Content of Weight Energy Layer 3a After CapacityDensity After Discharge Retention Ratio 300th Cycle (atomic %) After300th Cycle (Wh/kg) Example 13 53% 90.3% 182 Comparative 16% 30.1% 61Example 9

EXAMPLE 14

In the following, the present invention will be more fully described byExample 14 according to the second embodiment of the present invention.In Example 14, the capacities of the electrodes were designed as shownin Table 11 so as to satisfy the above formulas (IV) to (VI) forproducing a battery. TABLE 11 Positive Electrode Negative ElectrodeAdditional Li Capacity Capacity Capacity Example 14 500 mAh 563 mAh 63mAh Example 15 455 mAh 563 mAh 63 mAh Example 16 417 mAh 563 mAh 63 mAhExample 17 385 mAh 563 mAh 63 mAh Comparative 500 mAh 500 mAh  0 mAhExample 10

The electrodes were prepared based on the design of tile electrodecapacities shown in Table I l, and a battery was formed. Copper foilgraphite having a thickness of 100 μm in after compression and Si wereuse(d for the collector 1 a, carbon layer 2 a and lithium absorbingmaterial layer 3 a of the negative electrode shown in FIG. 1,respectively. After forming the lithium absorbing material layer 3 a, Liin a quantity shown in Table 11 was deposited thereon to make theaddition of Li. A lithium manganate mixture was employed for a positiveelectrode or cathode active material, and aluminum foil was employed fora cathode collector. A mixed solvent of ethylenecarbonate (EC) anddiethylcarbonate (DEC) (EC/DEC mixture ratio: 30 to 70 by volume), inwhich 1 mol/l (1M) of LiPF₆ was dissolved, was employed for anelectrolyte solution.

As Examples 15, 16 and 17, the electrodes were produced based on theelectrode design shown in Table 11 differently from the electrodestructure of Example 14. Otherwise, cylindrical secondary batteries wereformed in the same manner as in Example 14.

As comparative Example 10, the electrodes were designed as shown inTable 11, and a cylindrical secondary battery was formed with the samematerials and in the same manner as in Example 14 based on the electrodedesign shown in Table 11.

The electric properties of the above-described cylindrical secondarybatteries were examined by a charge/discharge tester. In Examples 14 to17 and Comparative Example 10, charging/discharging was carried out from2.5 to 4.2 V.

In all the Examples 14 to 17 and Comparative Example 10, the batteriesor cells were disassembled after the initial discharge and part of theelectrodes were cut out. Then, secondary ion mass spectrometry wascarried out to measure the Li content in the lithium absorbing materiallayer 3 a. Results of the tests are shown in Table 12. The Li content ofthe lithium absorbing material layer 3 a was 49 atomic-% at a dischargedepth of 100% in Examples 14 to 17, while that in Comparative Example 10was 16 atomic-%.

Besides, part of the same electrodes as used in Examples 14 to 17 andComparative Example 10 were cut out into a circular form with a diameterof 1 cm. After that, coin-shaped batteries were produced with the use ofLi metal as counter electrodes. Then, the positive electrodes wereallowed to charge/discharge from 2.5 to 4.3 V, and the negativeelectrodes were allowed to charge/discharge from 2.5 to 0 V at 0.1 mA.In the initial charge/discharge, a capacity of 5 mAh was observed inExample 14 and Comparative Example 10, a capacity of 4.55 mAh wasobserved in Example 15, a capacity of 4.17 mAh was observed in Example16 and a capacity of 3.85 mAh was observed in Example 17 at 4.3 V withrespect to the positive electrodes. On the other hand, with respect tothe negative electrodes, a capacity of 5.63 mAh was observed at 0 V inExamples 14 to 17, while a capacity of 5 mAh was observed in ComparativeExample 10.

Further, the batteries were continuously discharged and recharged untilthe 300th cycle in Examples 14 to 17 and Comparative Example 10. Thecapacity retention ratios after the 300th cycle are shown in Table 12.The capacity retention ratios were obtained using formula ( II).

Compared with Comparative Example 10 in which the Li content of thelithium absorbing material layer 3 a was 16 atomic-% at a dischargedepth of 100%, in Examples 14 to 17 where the Li content of the lithiumabsorbing material layer 3 a was 49 atomic-% at a discharge depth of100%, the capacity retention ratio after the 300th cycle rose by morethan 63%. As just described, Examples 14 to 17 proved that cyclecharacteristics were enormously improved when the lithium absorbingmaterial layer 3 a contained Li at a discharge depth of 100%.

Besides, Table 10 shows the weight energy density (Wh/kg) after the300th cycle in Examples 14 to 17 and Comparative Example 10. Referringto Table 12, the weight energy density was over 147 Wh/kg in Examples 14to 1 7, and it was improved by 93 Wh/kg or more as compared toComparative Example 10. Thus, it was confirmed that a high energydensity was achieved in Examples 14 to 17. TABLE 12 Li Content of WeightEnergy Layer 3a After Capacity Density After Discharge Retention Ratio300th Cycle (atomic %) After 300th Cycle (Wh/kg) Example 14 49% 94.6%168 Example 15 49% 95.1% 161 Example 16 49% 93.9% 152 Example 17 49%94.3% 147 Comparative 16% 30.1% 54 Example 10Industrial Applicability

In accordance with the present invention, the negative electrode hashigher capacity than that of the positive electrode, and the Li contentin a layer that is largely composed of lithium ion absorbing material iscontrolled to be 31 to 67 atomic-% at a discharge depth of 100% in orderto alleviate the expansion and contraction of the volume of the lithiumabsorbing material layer on the occasion of charging and discharging.Thus, the lithium absorbing material layer can be prevented from beingpulverized and flaking off. As a result, it is possible to obtain alithium ion secondary battery provided with both high weight energydensity and good cycle characteristics.

1. A lithium ion secondary battery comprising a positive electrodecapable of absorbing and desorbing lithium ions and a negative electrodeincluding a first layer that is largely composed of carbon and a secondlayer that contains an element to be alloyed with lithium, wherein thelithium content of the second layer is between 31 and 67 atomic-% at adischarge depth of 100%.
 2. The lithium ion secondary battery claimed inclaim 1, wherein the capacity of the negative electrode is higher thanthat of the positive electrode.
 3. The lithium ion secondary batteryclaimed in claim 2, wherein lithium in a quantity that satisfies thefollowing formulas (1) and (2) is electrically connected to the positiveelectrode or the negative electrode:Li=Cb(1−L _(c))+(M _(atom) ×L _(s)/(1−L _(s)))×Li _(capa)  (1);Li+Cat≦Cb+M _(atom) ×M _(capa)  (2); (wherein Li represents the capacityof Li electrically connected to the positive electrode or the negativeelectrode, Cb represents the capacity of active material contained inthe first layer, L_(c) represents the initial charging/dischargingefficiency of the first layer, M_(atom) represents the number of atomsof lithium absorbing material contained in the second layer, L_(s)represents the Li content in the second layer at a discharge depth of100%, Li_(capa) represents the capacity of a lithium atom, Catrepresents the capacity of the positive electrode, and M_(capa)represents the capacity of an atom of lithium absorbing materialcontained in the second layer).
 4. The lithium ion secondary batteryclaimed in claim 1, wherein the element to be alloyed with lithium is atleast one selected from Si, Ge, In, Sn, Ag, Al and Pb.
 5. The lithiumion secondary battery claimed in claim 1, wherein, as the element to bealloyed with lithium, Si and/or Sn is/are included.
 6. The lithium ionsecondary battery claimed in claim 1, wherein the first layer includesat least one selected from graphite, fullerene, carbon nanotube, diamondlike carbon, amorphous carbon, and hard carbon.
 7. The lithium ionsecondary battery claimed in claim 1, wherein the active material of thepositive electrode includes at least one compound selected from lithiumcobalt oxide, lithium manganese oxide, and lithium nickel oxide.
 8. Thelithium ion secondary battery claimed in claim 1, wherein the activematerial of the positive electrode includes lithium manganate.
 9. Amethod for using a lithium secondary battery comprising a positiveelectrode capable of absorbing and desorbing lithium ions and a negativeelectrode including a first layer that is largely composed of carbon anda second layer that contains an element to be alloyed with lithium,wherein the lithium content in the second layer of the negativeelectrode is made between 31 and 67 atomic-% on completion of discharge.10. The method for using a lithium ion secondary battery claimed inclaim 9, wherein the capacity of the negative electrode is higher thanthat of the positive electrode.
 11. The method for using a lithium ionsecondary battery claimed in claim 9, wherein the element to be alloyedwith lithium is at least one selected from Si, Ge, In, Sn, Ag, Al andPb.
 12. The method for using a lithium ion secondary battery claimed inclaim 9, wherein, as the element to be alloyed with lithium, Si and/orSn is/are included.
 13. A method for manufacturing a lithium ionsecondary battery comprising a positive electrode capable of absorbingand desorbing lithium ions and a negative electrode, involving the stepof, after forming the negative electrode including a first layer that islargely composed of carbon and a second layer that contains an elementto be alloyed with lithium, adding lithium in a quantity that satisfiesthe following formulas (A) to (D) to the surface of the positiveelectrode or the negative electrode:Cb+M _(atom) ×M _(capa)>Cat  (A);0.31≦L _(s)≦0.67  (B);Li=Cb(1−L _(c))+(M _(atom) ×L _(s)/(1−L _(s)))×Li _(capa)  (C);Li+Cat≦Cb+M _(atom) ×M _(capa)  (D); (wherein Li represents the capacityof Li electrically connected to the positive electrode or the negativeelectrode, Cb represents the capacity of active material contained inthe first layer of the negative electrode, L_(c) represents the initialcharge/discharge efficiency of the first layer of the negativeelectrode, M_(atom) represents the number of atoms of lithium absorbingmaterial being active material contained in the second layer of thenegative electrode, L_(s) represents the Li content in the second layerof the negative electrode at a discharge depth of 100%, Li_(capa)represents the capacity of a lithium atom, Cat represents the capacityof the positive electrode, and M_(capa) represents the capacity of anatom of lithium absorbing material being active material contained inthe second layer of the negative electrode).